The goal of the proposed work is to study the selectivity of synaptic connections formed in vitro by individual, identified neurons isolated from the CNS of the medicinal leech (Hirudo medicinalis). Identified sensory, motor and interneurons are easily isolated using simple procedures. The cells survive well in culture and can form chemical and electrical connections. Isolated cells will be placed together in pairs in vitro and recorded from to determine whether a synaptic connection has formed between the cells. This procedure will be repeated to test a variety of pairwise combinations of identified cells. The anatomy and physiology of the connections formed by the cells will be studied and compared with their in vivo counterparts. The synaptic preferences revealed by such isolated cells may provide insights into the way neurons choose their synaptic partners.